Review and Experimental Long Range Forecast for SW England. April 2021.

Published 19 April 2021.

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1. Influences.

Changes in sea temperature

Atlantic sea temperatures are generally warmer than climatology although areas to the west of Ireland are cooler due to lack of SW to W winds. The colder Northerly types of late have resulted in cooler values in the southern North Sea despite most of the North Sea being warmer than average. The Norwegian Sea, Baltic Sea and the Med are also mostly near or above normal. The tropical North Atlantic is forecast to remain slightly warmer than average through Summer and Autumn 2021 (See Met Office graphic below).

Colder sea temperatures in the East Pacific show a weakening La Nina with further noticeable warming having occurred in the west. Forecasts suggest that Pacific Sea Surface temperatures may remain in a weak La Nina or Neutral state through the remainder of 2021.

The North Atlantic Oscillation index (500hPa index shown below) has been negative recently (consistent with the blocked pattern and High pressure near the UK) and is forecast to remain slightly negative or neutral for a short time before becoming uncertain.

The positive NAO phase represents a stronger than usual difference in pressure allowing winds from the west to predominate. The position of the jet stream enables stronger and more frequent storms to travel across the Atlantic bringing more frequent milder and wetter weather types for NW Europe which in summer results in cooler temperatures.

For Met Office information about the NAO see https://www.metoffice.gov.uk/weather/learn-about/weather/atmosphere/north-atlantic-oscillation

Recent Climatology –  SW England

Up to the 18th temperatures averaged out around 7 Celsius which is about 2 degree below average for April and there have been an unusual number of frosty night/mornings. There has been 10% or less of the normal rainfall and probably more sunshine than usual too.

CPC temperature anomaly and rainfall total for 4 to 10th April 2021

Looking back at the last three months temperatures have recovered since the colder January and rainfall has been trending near ,then below average across SW England.

River flows and ground water levels in March 2021 were normal but trending lower across SW England. Details can be found in the March 2021 Hydrological summary PDF 

UK based data is available in The Hydrological Outlook which provides an insight into future hydrological conditions across the UK.  Detail can be found at http://www.hydoutuk.net/

River Flow from Climate Forecast.

Global Flood Awareness (updated) System. April forecast for the period April to July indicates slightly above normal flood risk for SW England. Blue above normal flood risk which probably mostly reflects the current ground water levels.

Reservoir levels across SW England were fairly high as of 11th April but trending lower than normal due to the dry first half of April

Atmosphere: Predictions.

A:  1: Stratosphere

North Pole stratospheric temperatures near normal or a little colder than average.

North Pole temperature at 10hPa and 30hPa

Since last month the polar stratospheric vortex has weakened and is moving towards the “summer” mode as can be seen in the 10hPa and 50hPa charts below for 1200UTC on the 18th and forecast for the 28th April.

: 2: Upper Troposphere

CFSv2 200hPa contours (top row) and CFSv2 anomaly (middle) and NMME (lower row) for May to August 2021 hint at a less mobile patterns across the Atlantic for May and June but more cyclonic patterns near the UK for July but more especially August.

B: Lower Troposphere:

Selection of May model solutions using April 2021 data are shown below, top row temperature anomaly and lower row precipitation anomaly.

CFS2, NASA, ECMWF and NMME

May data from WMO

Selection of forecast anomalies for May from WMO : Left to right WMO super ensemble, Brazil, UKMO, Canada, Germany, Japan, France, China, Russia. (see http://www.weather-info.co.uk/wxsvc/seaslatest.html for full sequence)

Selection of Summer (June to August) solutions using April 2021 data are shown below, top row temperature anomaly and lower row precipitation anomaly. The full set of graphics can be seen at http://www.weather-info.co.uk/wxsvc/seaslatest.html

CFSv2
NMME
NASA
ECMWF

Data from WMO

WMO super ensemble of 10 international models.
Washington
UKMO
Meteo France
Korea Met Agency
Canada Met Agency

ECMWF

German Met Service (DWD)

Brazilian Met Service
Bureau of Met Australia
China Met Agency.

C: Comparing the output of seasonal models for 2021 January, February and March based on December 2020 data for the UK and Ireland.

For graphics and details see Verification review 2020 Dec 2021 January February

Review of details looked for in seasonal forecast.
Temperature. For the three months together: England Wales and N Ireland were on aggregate above normal and Scotland below. Jan was cold and other months milder.

Rainfall. For the three months together: Above normal but the detail more varied especially the large areas of below average in March.

Sunshine Scotland totals above average but elsewhere below average although the month to month picture is more complex.

Pressure. South below and North above average. Higher pressure generally in March.

Results: Monthly detail poor in almost all models. DWD and to some extent ECMWF got the warming trend but all models failed to get below normal January followed by warmer Feb and Mar. Toulouse, NMME and CFSv2 got some idea of the trend in precipitation.


Scoring for the three month season will attempt to state good, fair, poor or no signal.

1. Russia: Temp poor . PPN poor .
2. USA – CFS2 : Temp good . PPN fair .
3. UKMO Contingency: Temp good . PPN good .
4. UKMO : Temp fair . PPN good . PMSL fair
5. USA – IRI : Temp no signal . PPN no signal .
6. KMA APCC : Temp good . PPN fair .
7. JMA : Temp fair . PPN poor . PMSL poor
8. NMME : Temp good . PPN fair .
9. WMO : Temp good . PPN fair .
10. BCC : Temp good. PPN good .
11. NASA – Temp fair . PPN fair .
12. Brazil: Temp fair. PPN poor. PMSL good.
13. CanSips : Temp fair. PPN good .
14. IMME : Temp good . PPN fair .
15. Copernicus Temp fair . PPN good . PMSL poor
16. CMCC Temp fair . PPN fair . PMSL poor
17: DWD Temp poor . PPN poor. PMSL poor
18. EC Temp fair PPN fair PMSL poor
18b JMA Temp poor. PPN poor. PMSL poor
19 UKMO Temp good . PPN good . PMSL
20. MF Temp fair . PPN fair . PMSL poor
21 NCEP Temp good . PPN fair . PMSL poor
22 JAMSTEC: only available for main seasons.
23 ECMWF (monthly) Temp: fair . PPN fair .

2: Forecast.

Remainder of Spring  (2021 May)

The overall indication for May is for slightly above average temperatures.

Rainfall most models suggest below average rainfall in May which would be remarkable given the drier April although the end of April may well become wet.

May climate: 1981-2010 average mean temperature 11 or 12°C. Average May rainfall roughly 70mm but 150mm over moors and 40mm in drier east each month.

Summer 2021  (June July August)  

Temperature near or slightly above average for the season with June possibly seeing the warmest anomalies and August the lowest.

Rainfall for the season uncertain but probably above average rain totals overall. The first half of summer may well be drier than average then turning wetter. Higher rain rates are typical of a warming atmosphere but may not imply an increase in the number of days with rain.

Summer climate: 1981 to 2010 average daily mean temperature 14 or 15°C in many areas to 16 or 17°C in main urban areas also locally east of the moors and more widely in Somerset. Maximum temperatures average 19 to over 21°C in similar areas. July often warmer than August. Average 1981 to 2010 rain 300mm over the moors, typically 200 to 250mm in many coastal and eastern areas. June often drier than July and July drier than August.

Autumn 2021  (September October November).

Milder than average also wetter especially early in the period. Hints that October could be a drier month.

Autumn climate: 1981 to 2010 average mean temperature 11 or 12°C but nearer 10°C in rural areas. Maximum temperatures average 14 to 16°C. November normally colder than October which is colder than September. Average 1981 to 2010 rain 300 to 400mm, 600 to 800mm over the moors but east of the moors and in lowland Somerset more like 200 to 300mm. September often drier than October or November.

Winter 2021 /22 (December January February) very limited data.


Milder overall and also wetter than average which implies below average snowfall.

Winter Climate: 1981-2010 average temperature values for lowland areas 7°C in the West and 5°C in the East. Rainfall; Dec and Jan typically wetter than February. 1981-2010 Autumn average 300-400 mm lowlands but 200-300 mm areas to E of Dartmoor. Snow climatology less than 5 days lying snow over lowland areas 5 to 10 hills, say hills around 200 metre elevation – one in three years have no lying snow.

3. Caution.

Experimental Long Range Forecasts do not have a good success rate. The data used for the above forecast summary can be seen at  here.

The attempt at a Regional Forecast for SW England aims to test whether such a forecast of temperature and rainfall variation from average can be made using numerical model data available on the internet. The forecast should not be used for any other purpose. A brief verification summary for the UK and Eire is routinely published at http://www.weather-info.co.uk/wxsvc/Verification.html or Click here for the Teignmouth and Dawlish summary

4. References.

SST anomaly NOAA Remote Sens. 20146(11), 11579-11606; doi:10.3390/rs61111579

IRI statistics: Mason, S.J. and L. Goddard, 2001: Probabilistic precipitation anomalies associated with ENSO. Bull. Amer. Meteor. Soc.82, 619-638.

UK climate details see: http://www.metoffice.gov.uk/public/weather/climate/

NMME information:   http://journals.ametsoc.org/doi/abs/10.1175/BAMS-D-12-00050.1

CFS2  info

GLOFAS Acknowledgement: Data were provided by the Global Flood Awareness System – GloFAS (http://www.globalfloods.eu/) of the European Commission Joint Research Centre and the European Centre for Medium-Range Weather Forecasts.
Reference: Alfieri, L., Burek, P., Dutra, E., Krzeminski, B., Muraro, D., Thielen, J., and Pappenberger, F.: GloFAS – global ensemble streamflow forecasting and flood early warning, Hydrol. Earth Syst. Sci., 17, 1161-1175, doi:10.5194/hess-17-1161-2013, 2013.

‘Copernicus Products’  as listed in the C3S or CAMS Service Product Specification or any other items available through an ECMWF Copernicus  http://climate.copernicus.eu

International seasonal monthly data from WMO

Stratospheric Diagnostics from Japan Met Agency and ECMWF via  Free University Berlin

Climate data from The Met Office UK and NCEP USA

Contains public sector information licensed under the Open Government Licence v3.0.

By Master0Garfield – Created using WikiProject Tropical cyclones/Tracks. The background image is from NASA [1]. The tracking data is from the National Hurricane Center’s Atlantic hurricane database, Public Domain, https://commons.wikimedia.org/w/index.php?curid=90697173

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